1. Field of the Invention
The present invention relates generally to subsurface decontamination or remediation, and more particularly, to the remediation of contaminated fine-grained sediments.
2. Description of Related Art
The National Research Council (NRC) (1994) published a book entitled, "Alternatives to Groundwater Cleanup", (National Research Council, National Academy Press, Washington, DC. (1994)) that summarizes the current state of the art in removing subsurface contaminants. In this book, the NRC arrived at the following conclusions. Current and developing technologies are cost-effective in remediating coarse-grained sediments (CGS) such as sands and gravels, but ineffective in removing contaminants from fine-grained sediments (FGS). The reason for the inability of present and developing technologies to remediate the FGS is primarily the very low hydraulic conductivity of FGS that effectively stops the flow of flushing fluids such as water, air, and steam from penetrating the FGS and flushing contaminants away. In contrast, the CGS possess hydraulic conductivities that are many orders of magnitude greater than FGS; consequently, they are readily flushed. Further, this book concludes that "while current technology can restore portions of the nation's contaminated ground-water sites to meet drinking water standards, total clean up at many sites is not feasible, even though such decontamination is required by federal and state laws."
Most contaminated subsurfaces are heterogeneous composed of bodies of CGS and FGS due to prior geological deposition mechanisms. CGS are contaminated by either capillary inhibition or advective groundwater transport. These mechanisms are effective in only bringing contaminants to the exterior surfaces of the FGS. The primary processes of contaminating the interior of the FGS is molecular diffusion, and sorption is the primary mechanism by which contaminants attach themselves to the hydrophobic organic components of the FGS. In addition, the inorganic clay components of the FGS are intrinsically negatively charged, and attract the positively charged toxic metal ions (TMI) onto the FGS surfaces. Random fluctuations in pH, temperature, redox potential, or other particle collisions in the groundwater can dislodge the TMI, polluting the groundwater.
Since current and developing subsurface technologies rely upon a flushing fluid to remove these hydrophobic contaminants, the advective removal of such contaminants is realized only in the CGS, leaving the contaminated FGS largely unaffected. As a consequence, the contaminants that have either adsorbed onto the organic matter onto the surface of the FGS or that have entered the interior by molecular diffusion, act as sources of contamination that slowly bleed contaminants back into the cleaned CGS. The NRC reported several case studies in heterogeneous sediments that were remediated by current technologies, only to be recontaminated in a few years to comparable or higher levels by the mechanisms of back-diffusion and desorption of contaminants from the fine-grained sediments.
It is the recontamination mechanisms of desorption and back-diffusion that renders subsurface remediation so time-consuming (many decades and centuries) and so expensive (many millions of dollars). The NRC concluded that the remediation of heterogeneous subsurfaces in both the unsaturated (vadose) zone and the saturated (groundwater) zone of contaminants to the stringent drinking water standards is not currently achievable by current and developing remediation technologies.
Existing subsurface remediation process technologies have a number of deficiencies. Typically, fuel hydrocarbons (FH) and halogenated hydrocarbons (HH) enter the subsurface as non-aqueous phase liquids (NAPL). Processes such as gravity flow and capillary forces transport NAPL through the vadose zone to encounter the saturated zone. A fraction of the contaminants will dissolve in the ground water to an extent governed by their intrinsic water solubility. Groundwater advective transport may carry them down-stream of the spill site, and advectively-based hydrodynamic dispersion will dissipate these contaminants over a larger volume in the CGS permeable zones. At a FGS interface, molecular diffusion and adsorption, not advection, are the mechanisms by which pollutants contaminate FGS. Similarly, the pollutants exit FGS by back-diffusion and desorption, not advection. Hence, if the FGS are contaminated, it is the very slow processes of molecular diffusion and desorption that make remediation of contaminated heterogeneous subsurfaces so expensive and time consuming, because the fast mechanism of advection does not play the dominant role of remediating FGS as in the case of the CGS.
The NRC book states "Underground environments vary widely, and many common contaminants have characteristics that make decontamination difficult. Because fluids move through irregular spaces between grains of sand and gravel, or through fractures in solid rock, contaminants often seep away from their sources in unpredictable ways. In some cases, contaminants are trapped in clay or microscopic pores in rocks too small for water to flush them out. These trapped contaminants can become long-term sources of pollution as they slowly diffuse into nearby groundwater."
FIG. 1, taken from FIG. 3--3, page 110, in Alternatives in Ground Water Cleanup, illustrates the time required for trichloroethene (TCE) to diffuse out of clay lenses at various penetration depths. The deeper the contaminants have penetrated, the longer will contaminants continue to diffuse out of clay lenses. In FIG. 1, TCE has penetrated to three example depths: 0.3 m, 0.6 m, and 1.2 m, and will persist in back-diffusing, for 20 years, 66 years, and well over a century, respectively.
The NRC has analyzed standard technologies and concluded that conventional pump-and-treat (P&T) could take a few years to tens, hundreds, or even thousands of years to effect remediation, depending upon the site. The committee concluded that P&T systems are beneficial because they can partially remove underground contaminants, keep them from migrating away from their sources, and contain and limit the size of the contaminated region. However, P&T technology is ineffective for cleaning up locations with significant amounts of solvents, precipitated metals, contaminants that have diffused into small pore spaces of the FGS or that adhere strongly to soils or other sediments. The committee found that increasing the pumping rate to remove mass from source areas is not efficient.
While enhanced P&T systems such as air sparging and in-situ bio-remediation can increase significantly the removal of contaminants and reduce treatment costs, such systems have the same limitations as conventional P&T in FGS and will not be able to fully restore sites with severe contamination.
The book points out that the site hydrogeology is a very important factor in determining the relative ease in remediating a contaminated aquifer. Homogeneous high permeability regions of sites are the easiest to mobilize and flush contaminants, whereas the tight zones in heterogeneous regions and fractured rock are most difficult. Strongly sorbed contaminants and those that have diffused into the clays are difficult to extract and continue to dissolve and/or diffuse into the groundwater.
The NRC investigated many enhanced pump and treat systems and alternative technologies that have been proven capable of shortening remediation time in permeable regions and reducing cost and concluded that these methods are greatly limited by the presence of low permeability zones and sites of strong sorption. These technologies include soil vapor extraction, in-situ bio-remediation, bio-venting, pulsed pumping, air sparging, steam-enhanced extraction, in-situ thermal desorption, flushing with surfactants or cosolvents, and injection of chemicals to transform contaminants in place. Physical containment of contaminants can prevent contaminant migration, but is not considered a permanent solution.
FIG. 2 illustrates the various mechanisms of clay contamination such as sorption of FH and HH contaminants onto the hydrophobic component of organic material at the surface of the FGS, the molecular diffusion of HH and/or hydrophilic contaminants into clays to the present penetration depth, and toxic metal cations attached to the negatively charged inorganic components of the FGS. The model shows a line 2 separating the uncontaminated FGS region 4 from the contaminated FGS region 6, which is separated from the CGS region 8 by line 10. When the permeable zones are cleaned, mechanisms of desorption, back-diffusion, and slow bleed of TMI act as source terms recontaminating the groundwater again, sometimes even to higher levels than previously encountered.